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Introduction to biomechanics in Frevialds Biomechanics of

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					IE 486 Work Analysis & Design II


  Lecture 13 – Intro to Biomechanics
         Dr. Vincent G. Duffy
       Thursday March 1, 2007
 IE486 – Lecture 13 - QOTD
Q.1. Is it reasonable to expect that the
‘whole’ is equivalent to the ‘sum of the
parts’? Yes/No?
Q.2 Which is more costly, low back injury
or CTD/upper limb?
Q.3 What is the recommended weight limit
for lifting in the given example?
– 5, 10, 50 lbs.?
 Recall from Ch. 10: Engineering
Anthropometry & Human Variability
Q.1. Is it reasonable to expect that the
‘whole’ is equivalent to the ‘sum of the
parts’?
– A tall person may have short arms. A person
  with long torso may have short legs.
– You can not measure one body part and
  extrapolate to know the remainder re: fit.
Administrative: see updated schedule with
  minor revisions highlighted in yellow
Today and after Spring Break
   What is the estimated cost of ignoring
issues related to the biomechanics of work?
  As of 1991, $27-56B (low back alone)
  according to Pope, et al. 1991.



  Other recent and related statistics are presented
  in NORA (National Occupational Research
  Agenda) and other documents on the NIOSH
  webpage. http://www.cdc.gov/niosh/nora/

  Related statistics are presented in Wickens et al.
  2004; Waters et al. 1993; 1999
Q.2 Which is more costly, low
    back injury or CTD?
Re: Upper-extremity cumulative trauma
disorders (CTDs).
Where repetitive hand and arm exertions
are prevalent, CTDs of the upper
extremities are common and can be even
more costly than low-back problems.
 Briefly discuss the ‘psychophysical’
 method of assessing static muscle
               strength.
Subjects adjust load upward and downward after
each trial in a simulated task situation until they
believe the load has reached THEIR maximum
capacity. It is be self-report (subjective rating).
It is suggested (according to Chaffin and
Andersson, 1991) that psychophysical methods,
even considering trouble with the method based
on motivation/cooperation, etc., may be the most
accurate method of estimating a person’s
strength.
And ECE 511 Prof. Hong Tan, Psychophysics.
Consider Figure 11.1. Find the moment
about the elbow for a single segment
biomechanical model of the forearm in which
the hand is holding a load of 25kg (rather
than 20kg).
Fundamentals. 1. A mass in motion (or at rest) remains in
motion (or at rest) until acted upon by an ‘unbalanced’
external force. 2. Force is proportional to the acceleration
of a mass (eg. At rest, use gravity). Any action is opposed
by a reaction of equal magnitude. That is why we can
assume that the sum of moments around the elbow is zero.
 Since we are assuming the ‘single-
 segment’ model, you can refer to the
 original figure 11.1 with modifications as
 follows.
The moment about the elbow is 46.98 Nm.
  This comes from the sum of moments around elbow =0
  sum M =16N*0.18m(unchanged – wt of forearm/hand) +25*9.8/2*0.36
  which is the new load divided by weight of each hand (by 2) multiplied by
  gravity and multiplied by distance from hand to elbow (unchanged).
Briefly discuss low back problems
    in relation to seated work.
Briefly discuss low back problems
    in relation to seated work.
Most people do not maintain an erect posture for long,
but adopt a slumped posture.
The slumped position produces wedging of disks in
lower back and can pressurize soft tissues in the spine
causing low-back MSDs.
What is the purpose of the NIOSH lifting equation?
What is AL and MPL and what is the difference
between them?
  According to the National Institute for Occupational
  Safety and Health (NIOSH, 1981), the purpose is to
  analyze lifting demands on low back.
  It allows the user of the analysis tool to establish a
  Recommended weight limit (RWL) for a specific task that
  nearly all healthy workers could perform for a substantial
  period of time without increased risk of developing lifting-
  related low-back pain.
  The AL is the action limit – a weight limit above which a
  small portion of the population may experience
  increased risk of injury whereas the Maximum
  permissible limit (MPL) is three times the action limit
  (AL).
   MPL is considered the weight limit at which most people
  would experience a high risk of back injury (for those lift
  conditions).
    What is the difference between the
original NIOSH lifting equation (1981) and
      the revised version from 1991?
 Eg. 1981 equation did not consider
 asymmetric lift.
 In 1991 the Lift Index (LI) is also used to
 quantify the degree to which a lifting task
 approaches or exceeds the RWL.
NIOSH Lifting Guide (Revised 1991)

NIOSH lifting equation is the ratio of load
lifted to RWL
                 LI = L / RWL
(a ratio; if LI>1, adjust task; task likely to pose
  increased risk for some workers; if LI>3, most
  workers at high risk for low back pain & injury)
For a given expected load to be lifted & given
  task, compute the RWL

RWL= LC x HM x VM x DM x AM x FM x CM
NIOSH Lifting Guide (Revised 1991)

compute the RWL
  – RWL= LC x HM x VM x DM x AM x FM x CM
LC=load constant
  – Max. recommended weight under optimal conditions
    eg. Symmetric lift, occasional lift, no torso twist, good
    coupling, <25cm vertical distance of lift
HM=horizontal multiplier
  – (moment) disc compression force increases as
    horizontal distance between load & spine increases.
  – Therefore, max. acceptable weight limit should be
    decreased from LC as horizontal distance increases
  NIOSH Lifting Guide (Revised 1991)
compute the RWL
   – RWL= LC x HM x VM x DM x AM x FM x CM
VM = vertical distance multiplier
   – Lifting from the floor is more stressful than lifting from
     greater heights.
   – Thus, allowable weight for lift is a function of the
     originating height of the load.
DM = distance multiplier
   – Physical stress increases as vertical distance of lift
     increases.
AM = asymmetric multiplier
   – Asymmetric lift involves torso twist and is more harmful
     to spine than symmetric lift. Therefore allowable load
     to be lifted should be reduced when lift includes
     asymmetric lifts.
NIOSH Lifting Guide (Revised 1991)


compute the RWL
  – RWL= LC x HM x VM x DM x AM x FM x CM
FM = frequency multiplier
  – Reflects effects of lifting frequency on
    acceptable lift weights.
CM= coupling multiplier
  – Difficulty of grab. Effected by whether load has
    handles.
   NIOSH Lifting Guide (Revised 1991)
compute the RWL
    RWL= LC x HM x VM x DM x AM x FM x CM
Components                    Metric          US
LC=load constant                23kg          51 lb.
HM=horizontal multiplier        25/H          10/H
VM= vertical distance multiplier (1-.003(V-75) 1-.0075(V-30)

DM= distance multiplier            .82+4.5/D   .82+1.8/D
AM= asymmetric multiplier          1-.0032A    1-.0032A
FM= frequency multiplier             see table 11.2
CM= coupling multiplier              see table 11.3
     Figure for NIOSH Lifting Analysis
       (consider QOTD 3. Compute the RWL)
                                        Outgoing
                                        J-conveyor




                          8”
Incoming conveyor   16”         62”
                          36”
H=16”
V=44”
D=18”
A=80degrees
F=3 lifts/minute
C=Good coupling
Job duration: 8 hrs/day
Wt. Lifted: 15 lbs.
Six multipliers that can be calculated to
get Recommended Weight Limit (RWL):
 HM= 10/H
 VM=1-.0075x(V-30)=
 DM=.82+1.8/D=.
 AM=1-.0032xA=
 FM= (from table)
 CM=( from table)
RWL=51xHMxVMxDMxAMxFMxCM
Six multipliers that can be calculated:
HM= 10/H = 10/16=.625
VM=1-.0075x(V-30)=1-.0075x(44-30)
  =.895
DM=.82+1.8/D=.82+1.8/18=.92
AM=1-.0032xA=1-.0032x80=.744
FM=.55 (from table at 3lifts per min. V>30”)
CM=1.0 (good coupling; from table)

RWL=51xHMxVMxDMxAMxFMxCM
   =
Six multipliers that can be calculated:

HM= 10/H = 10/16=.625
VM=1-.0075x(V-30)=1-.0075x(44-30)
  =.895
DM=.82+1.8/D=.82+1.8/18=.92
AM=1-.0032xA=1-.0032x80=.744
FM=.55(from table at 3lifts per min. V>30”)
CM=1.0 (good coupling; from table)

RWL=51xHMxVMxDMxAMxFMxCM
   =51x.625x.895x.92x.744x.55x1.0
    =10.74 (lbs)
               Lift index


LI = L/RWL = 15/10.74=1.4
some workers would experience an increase in
risk of back injury because the lift index is >1.0.
some precautions should be taken to minimize
the risk of injury, and the job may need to be
redesigned to lower the LI.

				
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